Power supply apparatuses include, for example, those that convert electric power generated by natural energy into grid electric power (hereinafter referred to as natural-energy power supply apparatuses) and those that convert electric power generated by energy other than natural energy into grid electric power (hereinafter referred to as nonnatural-energy power supply apparatuses).
The natural-energy power supply apparatus includes, for example, a solar power supply apparatus that converts power output from solar cells (solar panels). Examples of the solar power supply apparatus known in the art include those that enable grid-interconnected operation for outputting AC power in connection with a mains of grid (hereinafter occasionally abbreviated as a grid) and those that enable independent operation for outputting AC power independently of the grid.
The nonnatural-energy power supply apparatus includes, for example, a fuel cell power supply apparatus or a rechargeable battery power supply apparatus. Similarly to the aforementioned natural-energy power supply apparatuses, the nonnatural-energy power supply apparatuses also enable the grid-interconnected operation and the independent operation. With a constant maximum generated power at the independent operation, the nonnatural-energy power supply apparatus is capable of outputting electric power almost stably at any time. Moreover, for the fuel cell power supply apparatus, there has been known a fuel cell system in which multiple fuel cell power supply apparatuses are connected in parallel, to thereby increase the power generation efficiency of the fuel cell as a whole even when subjected to load changes (for example, see JP 2007-287567 A (PTL 1)).